Between the 1820s and 1880s, therapeutic explanation and practice in America were altered fundamentally. Traditional medical practices, based upon assumptions about disease shared by doctor and patient and oriented toward visibly altering the symptoms of sick individuals, were largely supplanted by therapeutic strategies grounded in experimental science that objectified disease while minimizing differences among patients. Concurrently, the bases of physicians' professional identity were also transformed. In the early nineteenth century professional identity was based upon behavior and a medical theory that stressed specificity in treating individual patients within diverse physical, social, and epidemiological environments. This commitment to specificity supported a belief in the different therapeutic needs of Northerner and Southerner, native born and immigrant, private and hospital patient, white and black. Toward the end of the century a new conception of professional identity, defined by an allegiance to knowledge generated and validated by experimental science and characterized by universalized diagnostic and therapeutic categories, was clearly in ascendance. This monograph, which I am preparing for publication, analyzes these changes as they occurred in the United States between 1820 and 1885, and particularly in Boston, Cincinnati, and New Orleans, by addressing three questions: (1) What changes occurred in medical therapeutic thought and practice, and what were the sources of these changes? (2) In what ways was medical care in different regions of America alike and different, how did these relationships change, and to what can these differences and changes be attributed? (3) How did changing patterns in the acceptance and rejection of therapeutic practices, and in regional variations in medical values and institutions, reflect and contribute to changes in physicians' professional identity? This study will assess historical documents using the techniques of social and cultural history, and include a computer analysis of manuscript medical case records.